Photopolymerizable negative-type photosensitive resin compositions are used in various applications, including photosensitizers for the production of color filters, overcoat photosensitizers, column spacers, light-shielding insulating materials, etc.
A typical photosensitive resin composition can be used to form a pattern by the following procedure. First, the photosensitive resin composition is applied to a substrate to form a coating film. A particular portion of the coating film is exposed to radiation through a photomask, and then the unexposed portion is developed to form the desired pattern.
Great efforts are currently undertaken to achieve improved yield per unit time in the processes using photosensitive compositions despite reduced exposure dose and time. To this end, an improvement in the photosensitivity of the photosensitive compositions is needed.
A representative approach to increase the photosensitivity of a photosensitive composition is either to use a photoinitiator with high photosensitivity or to increase the amount of a photoinitiator used in the photosensitive composition. The former case has the problem that the photoinitiator with high photosensitivity is relatively expensive, and the latter case has the problem that large quantities of sublimable impurities are generated during post baking to contaminate an oven or materials (e.g., liquid crystal) and components provided within an LCD panel.
In an attempt to introduce photopolymerizable functional groups into side chains of an alkali-soluble resin used in a photosensitive composition, a method has been proposed for photocrosslinking the alkali-soluble resin with an ethylenically un-saturated compound.
As the proportion of the photopolymerizable functional groups introduced into the alkali-soluble resin increases, the photosensitivity of the photosensitive composition is improved. However, the photopolymerizable functional groups are also introduced into add groups of the alkali-soluble resin to reduce the relative proportion of the add groups remaining in the alkali-soluble resin, resulting in poor developability of the photosensitive composition.
Under these circumstances, many methods have been developed to meet both photo-sensitivity and developability required in photosensitive compositions. For example, an excessively large amount of a monomer having an add group is added to increase the proportion of add groups in an alkali-soluble resin, thereby maintaining the add value at a sufficiently high level even after a number of photopolymerizable functional groups are introduced in the subsequent step. However, the alkali-soluble resin having a high add value suffers from low solubility in a solvent commonly used to introduce the photopolymerizable functional groups. This low solubility induces precipitation of the alkali-soluble resin during polymerization to make it impossible to attain a desired molecular weight of the alkali-soluble resin. Further, the increased proportion of the monomer having an add group leads to a relatively low proportion of a monomer added to impart other coating characteristics of a film, causing damage to the intended characteristics of the film.
A highly polar solvent can be used during polymerization to increase the solubility of the alkali-soluble resin with a high add value in a solvent, but it tends to impair the dispersion stability of a color filter during subsequent processing.
Thus, there is a strong need for a photosensitive resin that is highly photosensitive and resistant to heat and chemicals.